The pathogenic yeast, Candida albicans is a major constituent of the human flora with a plasticity that allows it to thrive both as a commensal organism and as a life-threatening pathogen. 90% of HIV-infected individuals develop oropharyngeal candidiasis (OPC), and oral Candida infections also affect 5% of newborns and 10% of elderly patients (2; 3). This fungal chameleon mediates changes in morphology, phenotype, and colony structure by responding to external cues, and these alterations endow C. albicans with traits that increase its virulence, enhance colonization, allow communication and resist anti-fungal drugs. The experiments contained in this proposal address key features of this plasticity that are paramount to C. albicans pathogenicity: how environmental signals and inter-species communication instigate phenotypic switching and pheromone signaling pathways, both of which are processes directly implicated in biofilm formation. My preliminary work shows that the white-opaque switch, which regulates both biofilm formation and mating, is increased in the presence of oxidative stress and also the commonly used anti-fungal drug Flucytosine. Given these results, in my first aim I will determine the rates of switching in C. albicans in response t oxidative stress, nitrosative stress and anti-fungal drug exposure. These factors are common challenges to C. albicans in the clinical setting, and as such, several clinical isolates will be analyzed. I will also identify the response pathway by which they increase phenotypic switching. I hypothesize that the thioredoxin pathway and Rad53 pathways are involved, so they will be tested using gene deletion and over-expression strains for TRX1, TRR1, CAP1, and RAD53. My second aim will address inter-species events between Candida albicans and the fungal and bacterial members of the human oral microbiome. Results from my laboratory shows that pheromone signals from other Candida species induce biofilm formation by C. albicans, which I will further investigate in this aim (14). First, I will determine if signaling between Candida species occurs at physiologically-relevant pheromone concentrations using a set of C. albicans strains I have constructed that secrete pheromones native to these other species in place of its own. I anticipate that my experiments will show that both mating and biofilm formation occur in response to signaling via several Candida species' pheromones, implicating inter-fungal signaling in pathogenesis. Furthermore, I will directly co-culture C. albicans with known bacterial members of the oral microbiome, including Streptococcus gordonii, which secretes high levels of hydrogen peroxide, a known stressor and switch inducer (43). Such results from my second aim will be supported by my first aim. Understanding the white-opaque switch and inter-species signaling events will allow the development of new treatments for preventing the often debilitating, and sometimes even fatal, infections associated with this fungal pathogen. PUBLIC HEALTH RELEVANCE: Candida albicans is an important part of the human flora, existing as both a commensal and pathogenic species, using changes in phenotype to evade host immune system responses and inhabit diverse niches. This proposal will test how C. albicans transitions to the opaque phenotype, which is important to successful infection, and the influence of inter-species signaling on C. albicans colonization and biofilm formation. These factors are key components in C. albicans ability to cause oral and vaginal thrush, denture stomatitis, and systemic infection, especially in immune compromised individuals.